South West Water (SWW) Cornborough, UK, WWTP is a new plant designed and built by Pattersen Candy, UK. Cornborough is a coastal community that experiences differing flows between tourist and non-tourist seasons. The plant itself is of the latest activated sludge design and incorporates exceptionally deep (8M) aeration tanks, with fine bubble diffusion aeration. The plant began to receive flows in the summer of 2002, and a problem of increased filamentous growth occurred in the colder months when sludge retention time (SRT) was held high to keep F/M under control. South West Water management requested that we trial a Royce Technologies Model 7700 SRT controller in order to attempt to get SRT under control with the fluctuating flow conditions and to maintain effluent consent. It was also believed that the controller should assist management in optimising the plant's operational capability with regard to process control and sludge management.
The plant is the first European WWTP to adopt automatic SRT control. It fulfilled all expectations, and more. It is linked to the plant scada system, which makes the ultimate decision whether to waste on a timed basis or to waste from the command of the controller; the latter is the scada's 'quality mode'. The Royce terminology for giving the scada the wasting control is 'intermittent control'. Once the customer was comfortable with the controller the scada was switched into the 'quality' wasting mode and left there. The controller has told the scada when, and how much to waste ever since. The controller is as easy to use as a domestic airconditioner. It is fail-safe because it communicates with the plant's scada system that initiates the wasting (surplusing) pump.
Since switching to automatic (quality) wasting it has been possible to adjust wasting times to suit the working hours of the operators. Before adopting automatic wasting, the operators would arrive at 7am, program the scada with the daily waste target based on the previous day's lab results and wasting would begin. Of course now wasting is able to start much earlier in the day - around 5 am. This brings forward the end of day's wasting requirement, making it easier to make plant adjustments throughout the day and before the operators go home. The controller runs right through the weekend in automatic. There are no longer any problems running the sludge holding tanks down on a Friday afternoon to avoid on-site sludge storage problems. The Friday wasting pattern is programmed to deal with this in conjunction with the operator's early finish on Fridays. Staff can now safely leave the plant with less knowledgeable operators if need be, and there is no need to call the plant up as often to check that the correct amount of sludge has been wasted that day. The Royce controller simply gets on with the job twenty-four hours a day, seven days a week.
Previously, in manual control, the plant's own daily lab samples of MLSS and RAS TSS were used to calculate a daily sludge volume to be wasted. This method required skill, experience and good judgement in order for the correct food to mass ratio to be achieved. The Royce SRT Controller has taken the guesswork out of this function by using a 15-minute rolling average of both MLSS and RAS. Every fifteen minutes the controller calculates the actual SRT of the plant, at the next pre-programmed wasting cycle it has the scada switch the wasting pump on for a period aimed at maintaining the daily waste flow average for the plant such that the ideal SRT for the plant is maintained.
When originally switched to automatic control the controller calculated the plant had been running an actual SRT of 5,7 days under manual control. We set the controller to a target of 3,7 days. Initially the controller wasted just over 1000 m3/day (under manual control the plant had been wasting 850 m3/day). Over the next week, as the sludge age approached the 3,7 day target, the controller began to suggest that a 3,3 day target would be best, so our operators changed the target to 3,3 days.
The Royce controller has freed up operator time while it and the scada system are in automatic control of the wasting and staff no longer have to wait for the lab results to calculate a daily wasting target. The automation of this critical process has resulted in much lower swings in mixed liquor (from 400 mg/l to 100 mg/l), resulting in the complete elimination of filamentous organisms. And now, the plant returns filtrate from the thickeners to the RAS line on a regular basis throughout the day starting at 7 am. There used to be a large wasting hit at 7 am when the operators arrived and opened the return valve sending the RAS down from 4000 mg/l to around 1000 mg/l. Since having the controller on line this event is seriously reduced, as it begins wasting three hours earlier than the operators were able to previously.
SWW is very pleased with the Royce controller despite initial reservations about its ability to reliably control sludge wasting, especially during weekends or unmanned periods. However once it was clear the controller was as capable as everybody had hoped, it was switched onto automatic and it has remained there ever since, including weekends. There were also initial concerns by the Black & Veatch representative, but now that it has been working so well over the past months, he is also impressed. In his opinion this controller is going to change the way wastewater treatment plants operate in the future.
In conclusion, the benefits had from installing the Royce Model 7700 controller included:
* Wasting can automatically take place outside of normal work hours, without experienced personnel on site.
* Seven day automatic wasting has allowed optimal use of the sludge holding facilities and better scheduling for third party sludge hauling contractors.
* Less chance of human error during the calculation of daily wasting regimes.
* Plant MLSS stability has improved resulting in less reactive control strategies during dry spells or rain events because the controller reacts to special events as they occur 24/7.
* The use of the controller eliminated filamentous growth in this plant.
* The Royce controller and suspended solids analysers provide a comprehensive database of historical trends, providing continual detailed knowledge about how the plant is performing, compared to daily lab tests that only provided daily snapshots.
* The controller has reduced time-consuming, and costly laboratory work for the operators.
For more information contact Peter Fleming, Action Instruments SA, 011 403 2247.
Richard Thompson is a wastewater treatment plant supervisor with South West Water, and the Royce Controller is available locally from Action Instruments SA.
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